PROJECT SUMMARY Historically, glia, and oligodendrocytes in particular, have been viewed as providing much needed support to neurons but not otherwise as being active players in neural circuits. In contrast to this view, it is now increasingly appreciated that myelin may be dynamically regulated throughout life, with neuronal activity being one of the drivers of myelin changes. This raises the exciting prospect that activity-modulated, or adaptive, myelination could be an import mechanism for either modulating neural circuitry or directing glial support to axons with the greatest metabolic demand. In spite of this increasing awareness of the link between neural activity and myelin plasticity, our understanding of exactly how OPCs and oligodendrocytes respond to neuronal activity remains rudimentary. There is an increasing consensus that activity can promote both the proliferation of OPCs and their differentiation into myelinating oligodendrocytes, though the exact sequence of events remains unclear. The responses of mature oligodendrocytes to neuronal activity are less well understood, though it has been recently shown that they can modulate expression of transporters and perhaps alter the thickness of their myelin sheaths in response to neuronal activity. To date, there has been no comprehensive analysis of the OPC and oligodendrocyte transcriptional response to activity. This represents a large gap in our knowledge as an understanding of oligodendroglial activity-regulated genes would provide key insight not only into the genes regulating OPC proliferation and differentiation in the adult CNS but also into aspects of the mature oligodendrocyte response that may not be easily detected by histological methods alone. In Aim 1 of this project we will make use of our recently published AAV-DREADD approach in conjunction with a transgenic method for nuclear purification (?INTACT?) to define the transcriptional responses of both OPCs and mature oligodendrocytes to neuronal activity in the adult brain. This will provide the first comprehensive activity-regulated transcriptome database for myelinating cells. In Aim 2, we will use an AAV approach to both up-regulate and disrupt candidate activity-regulated genes in OPCs, defining the contribution of individual genes to oligodendrogenesis in the adult CNS. Overall, this project is innovative because it uses state of the art approaches to both define the glial transcriptional response to neuronal activity and determine the functional role of individual genes in an in vivo setting. This contribution will be significant because it will provide a framework for understanding how OPCs and oligodendrocytes respond to activity as well as identifying individual target genes that regulate myelination in the adult CNS relevant to both plasticity and, potentially, repair.